Like virus, bacteria, fungi and algae, protozoa is distinct from higher organisms in that they lack the specialized organization of the cells of higher order living systems. Protozoa, which occur in the kingdom Protista, is a microbial species sometimes denoted a predator, since it derives its nutrition from ingestion of bacteria. Many widespread diseases are caused by protozoan pathogens, such as malaria, leishmaniasis and trypanosomiasis. The last mentioned is caused by infections with species of the protozoan genus Trypanosoma. American trypanosomiasis, or Chaga's disease, is caused by T. cruzi, which is usually transmitted to humans by infected triatomid bugs, while African trypanosomiasis, or African sleeping sickness, is caused by infections of Trypanosoma brucei rhodiense and T. b. gambiense. More specifically, African sleeping sickness (for a review see Brun, R. (1999) Karger-Gazette 63, 5–7) is a devastating disease that has got its name from the comatose condition at the final stage of the disease. Without treatment, the patient dies within a couple of months to several years after infection. The severe comatose condition described occurs when these parasites from mainly being circulating in the bloodstream also invade the central nervous system.
T. brucei is spread between its mammalian hosts by tsetse flies. The parasite goes through many different life cycle stages in the mammalian host as well as in the fly (Vickerman, 1985). When the trypanosomes enter the mammalian host through a tsetse bite, they start to proliferate as long slender bloodstream forms. While the disease progresses, more and more trypanosomes are converted to short stumpy forms that are unable to proliferate but in contrast to the long slender forms can be transmitted to a new tsetse fly. In the fly, the trypanosomes go through some further developmental stages before they are passed on to a new mammalian host.
Most drugs effective against T. brucei do not cross the blood-brain barrier and are therefore useless as soon as the trypanosomes have invaded the CNS. Currently, the only drugs effective against a late stage T. brucei infection are difluoromethylornithine (DFMO) and arsenicals such as Melarsoprol. The arsenicals have severe side effects and the patient often dies from a drug-induced encephalitis. DFMO is less toxic but very expensive to make and is only effective against T. b. gambiense. Due to economic reasons, it is not produced anymore.
In order to find alternative treatments, purine metabolism has for a long time been a hot subject in the field of trypanosomes and related organisms (reviewed in Hassan, H. F. and Coombs, G. H. (1988) FEMS Microbiol. Rev. 54, 47–84). It has previously been established that trypanosomes lack the ability to form purines de novo and therefore need hypoxanthine, adenine or guanine which are salvaged through various phosphoribosyltransferases. Any of these three bases is fine since the trypanosomes, like most organisms, have all the enzymes needed to interconvert IMP, AMP and GMP. Nucleosides (adenosine, inosine and guanosine) can also be used, but they are usually split into sugar and base before their salvage (Pellé, R., Vern, L. S., Parkin, D. W. (1998) J. Biol. Chem. 273, 2118–2126 Proc. Am. Ass. Cancer Res. 22, 352).
However, much less attention has been focused on the synthesis of pyrimidines in trypanosomes (for a review see Hassan, H. F. and Coombs, G. H. (1988) FEMS Microbiol. Rev. 54, 47–84). It is known that T. brucei is fully capable of making UTP de novo and by salvage of uracil. Since there is no information whether T. brucei synthesizes CTP de novo or by salvage of cytidine or cytosine, up to now, it has not been possible to develop drugs based on the control of CTP levels.